To Shield or Not to Shield: Application of Bismuth Breast Shields

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1 Cardiopulmonary Imaging Opinion Colletti et al. Bismuth Breast Shields Cardiopulmonary Imaging Opinion FOCUS ON: Patrick M. Colletti 1 Orlando A. Micheli Kai H. Lee Colletti PM, Micheli OA, Lee KH Keywords: bismuth breast shield, breast CT, breast phantom, breast radiation, radiation reduction DOI: /AJR Received September 18, 2012; accepted after revision October 26, All authors: Department of Radiology, University of Southern California Keck School of Medicine, LAC+USC Medical Center, 1200 N State St, Los Angeles, CA Address correspondence to P. M. Colletti (Colletti@usc.edu). AJR 2013; 200: X/13/ American Roentgen Ray Society To Shield or Not to Shield: Application of Bismuth Breast Shields OBJECTIVE. The purposes of this essay are to describe the effects of bismuth breast shielding on radiation exposure of the breast and posterior chest wall and to present arguments for and against the use of breast shields. CONCLUSION. Breast tissue may receive substantial radiation doses during CT examinations. Bismuth shields effectively reduce breast exposure at the expense of increased noise and artifacts. Because bismuth shields reduce radiation transmission in all directions, posterior-to-anterior irradiation results in wasted exposure of posterior tissues. Similar breast radiation reductions can be achieved without shielding by globally reducing tube current. In general, more advanced methods of reducing exposure, including dose modulation and iterative reconstruction techniques, are superior if available. Why Is It Important to Minimize Breast Radiation? Substantial doses of ionizing radiation are delivered to the body during CT, and the lifetime attributable risk of fatal cancer due to pelvic, abdominal, or chest CT has been predicted to range from 25 to 33 cases per 100,000 examinations [1]. Although many tissues may be susceptible to ionizing radiation, breast tissue is among the most vulnerable. Study of atomic bomb survivors revealed that higher doses of radiation were correlated with greater risk of development of breast cancer, and women who had been exposed during childhood or adolescence were especially vulnerable to this risk [2]. Apparently significant increases in breast cancer have also been reported among women who undergo the multiple radiographic examinations associated with scoliosis [3, 4]. The risks of breast cancer induction with annual or semiannual screening mammography have been projected without specific supporting evidence [5]. The weighting factor for determining radiation dose for breast tissue, which had decreased from 0.15 in 1977 to 0.05 in 1990, increased to 0.12 in 2007 owing to an increase in predicted radiosensitivity [6, 7]. In addition to being relatively radiosensitive, breast tissue may receive a large dose of radiation during diagnostic studies. During pulmonary CT angiography for pulmonary embolism, the mean glandular dose to breast tissue may range from 20 to 60 mgy, and the inferior aspect of the breast may receive approximately mgy during abdominal CT [8]. Despite the relatively large radiation dose, CT is a generally available, important diagnostic tool; the overall number of CT examinations increased from 3 million per year in 1980 to 62 million per year in 2006 [9]. CT of the chest accounts for many of these scans, and it is estimated on the basis of 2010 National Health Interview Survey information that approximately 10 million individuals underwent chest CT that year [10]. It has also been suggested that repeated use of CT for a single patient may increase that person s cumulative risk of development of cancer over the risk incurred by a typical patient [11]. On the basis of the overall predicted radiosensitivity of breast tissue, the potential dose of radiation the breast receives during CT, and the overall frequency with which the breast may be irradiated during medical care, reliable and practical means of reducing radiation to the breast are important. How Can Breast Radiation Exposure From CT Be Reduced? Besides avoidance of direct irradiation of the breast [12], there are a number of traditional methods for reducing radiation exposure of breast tissue in the CT primary beam [13 15]. Although tube current modulation methods may reduce overall glandular breast AJR:200, March

Colletti et al. and lung tissue radiation, such techniques may become less effective as the diameter of the patient increases and can even increase the dose in larger patients [16, 17].

2 Colletti et al. and lung tissue radiation, such techniques may become less effective as the diameter of the patient increases and can even increase the dose in larger patients [16, 17]. Among the most promising techniques for breast exposure reduction is organ-based tube current modulation [18]. Selective shielding was introduced as a potential means of reducing radiation dose to radiosensitive areas during CT scans [19 22]. Although other materials have been tested [23], bismuth (atomic number 83 compared with lead at 82) typically has been used for this purpose. Breast Shield Phantom A phantom model was created to show the effects of bismuth breast shielding on the radiation exposure of the breast and posterior chest wall. A cm water-filled phantom with two attached symmetric breast phantoms constructed of candle wax ( 77 HU) to simulate fat and a fibroglandular cone simulated from a bolus material tissue simulator (47 HU) (Superflab, Mick Radio-Nuclear Instruments) was manufactured to simulate extremely dense breasts. The radiation doses were measured with a patient skin dosimeter (Unfors PSD, Unfors). This dosimeter has two mm solidstate radiation detectors. One detector was inserted into the midsection of the breast phantom to simulate the mean glandular dose. The second detector was taped to the back surface of the phantom for estimation of the posterior skin entrance dose. The bismuth shield (AttenuRad, F&L Medical Products) consisted of 1-mm-thick bismuth impregnated in synthetic rubber with 0.25 inch of foam spacer between the bismuth and the skin. The phantom was scanned with a 16- MDCT time-of-flight PET/CT system (Gemini, Philips Healthcare) operating in the CT mode. The parameters were set to 120 kvp, mm acquisition width, pitch, and 0.75-second tube rotation. The console computer selected the tube current for the desired tube current time setting per rotation. Images were reconstructed to 2 mm thick, and a B-kernel mildly smoothing reconstruction algorithm was applied. Three scans were acquired to obtain the average doses with various combinations of dose modulation techniques with and without the breast shield. Simultaneous radiation data were recorded in the breast phantom and posterior to the torso water phantom. Image noise was measured as the SD of attenuation (HU) in selected anterior and posterior regions of interest in the water phantom. The phantom results are shown in Figure 1 and compared in Table 1. The Argument for Breast Shields Breast shields are clearly effective in reducing breast radiation exposure. In a most simplistic approach, in the absence of adaptive dose modulation, breast shields may be applied with the expectation of significant breast dose reduction, on the order of 30% Fig. 1 Water-filled phantom with breast phantoms attached and diode sensors in breast phantom and on posterior surface. All CT scans were obtained at 120 kvp with fixed tube rotation time of 0.75 seconds. Expected relative differences in image noise are apparent. A, 80 ma, dose modulation off. B, 80 ma, dose modulation off, breast shield on for scan. C, 60 ma, dose modulation off. D, 80 ma, z-axis dose modulation on. E, 80 ma, z-axis dose modulation on, breast shield on for scan. F, 80 ma, z-axis dose modulation on, breast shield on for scout image and scan. 504 AJR:200, March 2013

3 Bismuth Breast Shields TABLE 1: Comparison of Exposure Reduction and Image Noise Without and With Breast Shielding No dose modulation Technique 120 kvp, 80 ma a 120 kvp, 80 ma a 120 kvp, 60 ma a Dose modulation Off Off Off Breast shield Off On for scan only Off Volume CT dose index, reported (mgy) Midbreast radiation (mgy) Posterior radiation (mgy) Anterior noise (SD) Posterior noise (SD) With z-axis dose modulation Technique 120 kvp, 80 ma a 120 kvp, 80 ma a 120 kvp, 80 ma a Dose modulation On On On Breast shield No shield On for scan only On for scout image and scan Volume CT dose index (reported) (mgy) Midbreast radiation (mgy) Posterior radiation (mgy) Anterior noise (SD) Posterior noise (SD) a Fixed tube rotation time of 0.75 seconds. as shown in Figures 1A and 1B and Table 1. Even with z-axis dose modulation, bismuth shielding specifically applied after acquisition of the scout image can offer breast dose reductions on the order of 37%, as in Figures 1D and 1E and Table 1. Results may vary depending on the specific adaptive dose modulation method applied. A radioprotective brassiere composed of bismuth and latex has been found to attenuate the delivered radiation dose by 52.4% [24]. Use of bismuth shields in girls 2 months to 18 years old had similar results: a 29% dose reduction and no significant artifacts seen in breast or lung tissue at MDCT [25]. Experiments with both human participants and phantoms to respectively determine the effectiveness of bismuth for reducing radiation doses to both superficial and deep glandular breast tissues revealed a 17% decrease in dose to the deep glandular tissue during MDCT with image diagnostic quality still deemed suitable by a thoracic radiologist [26]. Hulten et al. [27] evaluated coronary CT angiography image quality in two groups of 36 women matched for age and body mass index who underwent imaging either with or without a breast shield. Those investigators concluded that for coronary CT angiography of women, breast shields slightly increased noise but did not [significantly] negatively impact signal, signal/noise, quality or interpretability [27]. The Argument Against Breast Shields As simple and as inexpensive ($154 for a midsize breast or large shield and foam offset) as bismuth breast shields are to use [27, 28], they may not be completely without tangible risk. However, like other objects (e.g., foam positioning pads, cassettes, monitors, transducers, and MRI coils) in the radiology environment that can spread infection [29], bismuth shields are covered with a porous material that is not readily sanitized. Although bismuth breast shields reduce the radiation dose to the breast, they have caused beam-hardening artifacts and increased noise (measured as the SD of attenuation in selected regions of interest) when used for scans of patients [30] and anthropomorphic phantoms [31]. For example, a 37% increase in image noise related to bismuth shielding without dose modulation is shown in Figures 1A and 1B and Table 1. Similarly, a 33% increase in noise is shown on z-axis dose-modulated images with the addition of bismuth shielding, as in Figures 1D and 1E and Table 1. Although in the simplest CT acquisition mode bismuth shields may be applied with little forethought, the addition of adaptive dose modulation can lead to less predictable results [32, 33]. For example, as shown in Figures 1E and 1F and Table 1, when z-axis dose modulation is added, if the bismuth shield is applied before the scout views are obtained, the added attenuation is adjusted for with increased tube current and an associated increase in volume CT dose index exposure of 20% and a measured increase in midbreast dose of 29%. On the other hand, as Figure 1E shows, the combination of z-axis modulation and breast shield application after acquisition of the scout image can result in rather noisy images. In addition, relatively minor geometric inconsistencies in shield placement can lead to variations in dosimetry and artifacts [34]. Because the shield blocks only primary x- ray beams coming from the anterior direction, the beams projecting from the posterior direction still provide the full dose of radiation to sensitive tissues such as breast and lung, but their attenuation on passing through the shield increases noise throughout the image [35 39]. To compare the use of bismuth breast shielding to global reduction of tube current, Figures 1B and 1C and Table 1 compare phantom 80-mA breast-shielded images with globally reduced 60-mA images obtained without shielding. The 60-mA image has an 11% higher absorbed dose with a 19% AJR:200, March

4 Colletti et al. reduction in noise compared with the 80-mA shielded images. One study that compared bismuth shielding, organ-based tube current modulation, and a global reduction of tube current to match the dose reduction accomplished by bismuth shielding [40] showed that all three methods achieved a similar reduction in dose but differed in the quality of the image produced. Both global current reduction and shielding resulted in an increase in image noise, and shielding also yielded streak and beam hardening artifacts. Recommendations for Use of a Bismuth Shield to Reduce Breast Exposure During CT If no other method of CT dose reduction is used, application of a bismuth shield will effectively reduce breast exposure by 30% or more. The use of bismuth breast shields is associated with increased noise and artifacts. CT attenuation values measured in bismuth-shielded regions are high compared with the values in unshielded regions. Bismuth breast shields are associated with some wasted radiation in that whereas anterior exposure is substantially reduced, posterior exposure is only minimally lower and the shield reduces the transmission of useful photons in both the anteroposterior and the posteroanterior directions. Equivalent reductions in breast radiation can be achieved by global reduction of tube current. Such a strategy has the advantage of lower radiation exposure throughout the imaged volume with the cost of a similar noise increase extending beyond the volume that would be affected by a bismuth shield. The use of bismuth breast shields with z- axis dose modulation in CT may yield similar reductions in breast dose. Breast shields should not be applied before the scout planning image is acquired. There is potential for unpredictable results, depending on the specific technique applied, especially if real-time x-y adaptive dose modulation is actively adjusting exposure for attenuation during scanning. Application of z-axis, x-y adaptive, or organ-based tube current modulation is generally superior to use of bismuth shields for CT breast dose reduction. Whenever possible, z-axis tube current modulation should be used. 257: Land CE, Tokunaga M, Koyama K, et al. Incidence of female breast cancer among atomic bomb survivors, Hiroshima and Nagasaki, Radiat Res 2003; 160: Hoffman DA, Lonstein JE, Morin MM, Visscher W, Harris BS 3rd, Boice JD Jr. Breast cancer in women with scoliosis exposed to multiple diagnostic x rays. J Natl Cancer Inst 1989; 81: Doody MM, Lonstein JE, Stovall M, Hacker DG, Luckyanov N, Land CE. Breast cancer mortality after diagnostic radiography: findings from the U.S. Scoliosis Cohort Study. Spine (Phila Pa 1976) 2000; 25: Yaffe MJ, Mainprize JG. Risk of radiation-induced breast cancer from mammographic screening. Radiology 2011; 258: [No authors listed]. The 2007 recommendations of the International Commission on Radiological Protection, ICRP publication 103. 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Radiology 1997; 205: Hopper KD, Neuman JD, King SH, Kunselman AR. Radioprotection to the eye during CT scanning. AJNR 2001; 22: Hohl C, Wildberger JE, Suss C, et al. Radiation dose reduction to breast and thyroid during MDCT: effectiveness of an in-plane bismuth shield. Acta Radiol 2006; 47: Mukundan S, Wang PI, Frush DP, et al. MOSFET dosimetry for radiation dose assessment of bismuth shielding of the eye in children. AJR 2007; 188: Parker MS, Kelleher NM, Hoots JA, Chung JK, Fatouros PP, Benedict SH. Absorbed radiation dose of the female breast during diagnostic multidetector chest CT and dose reduction with a tungsten-antimony composite breast shield: preliminary results. Clin Radiol 2008; 63: Hopper KD. Orbital, thyroid, and breast superficial radiation shielding for patients undergoing diagnostic CT. Semin Ultrasound CT MRI 2002; 23: Fricke BL, Donnelly LF, Frush DP, et al. 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5 Bismuth Breast Shields Cardiol 2012; 19: Geleijns J, Salvado Artells M, Veldkamp WJ, Lopez Tortosa M, Calzado Cantera A. Quantitative assessment of selective in-plane shielding of tissues in computed tomography through evaluation of absorbed dose and image quality. Eur Radiol 2006; 16: Coursey C, Frush DP, Yoshizumi T, Toncheva G, Nguyen G, Greenberg SB. Pediatric chest MDCT using tube current modulation: effect on radiation dose with breast shielding. AJR 2008; 190:244; [web]w54 W Leswick DA, Hunt MM, Webster ST, Fladeland DA. Thyroid shields versus z-axis automatic tube current modulation for dose reduction at neck CT. Radiology 2008; 249: Kalra MK, Dang P, Singh S, Saini S, Shepard JA. In-plane shielding for CT: effect of off-centering, automatic exposure control and shield-to-surface distance. Korean J Radiol 2009; 10: McCollough CH, Wang J, Berland LL. Bismuth shields for CT dose reduction: do they help or hurt? J Am Coll Radiol 2011; 8: Geleijns J, Wang J, McCollough C. The use of breast shielding for dose reduction in pediatric CT: arguments against the proposition. Pediatr Radiol 2010; 40: AAPM Board of Directors. AAPM position statement on the use of bismuth shielding for the purpose of dose reduction in CT scanning. Policy PP-26-A. pdf. February 7, Accessed September 16, 2012 FOR YOUR INFORMATION Mark your calendar for the following ARRS annual meetings: April 14 19, 2013 Marriott Wardman Park, Washington, DC May 4 9, 2014 Manchester Grand Hyatt San Diego, San Diego, CA April 19 24, 2015 Toronto Convention Centre, Toronto, ON, Canada 38. Vollmar SV, Kalender WA. Reduction of dose to the female breast in thoracic CT: a comparison of standard-protocol, bismuth-shielded, partial and tube-current-modulated CT examinations. Eur Radiol 2008; 18: Wang J, Duan X, Christner JA, Leng S, Grant KL, McCollough CH. Bismuth shielding, organ-based tube current modulation and global reduction of tube current for dose reduction to the eye in head CT. Radiology 2012; 262: Wang J, Duan X, Christner JA, Leng S, Yu L, Mc- Collough CH. Radiation dose reduction to the breast in thoracic CT: comparison of bismuth shielding, organ-based tube current modulation and use of a globally decreased tube current. Med Phys 2011; 38: AJR:200, March

Comparative Analysis of Radiation Dose and Image Quality Between Thyroid Shielding and Unshielding During CT Examination of the Neck

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